1. Field of the Invention
This invention relates to improvements in vacuum-tube audio power amplifier circuit topology.
2. Description of the Prior Art
Examples of prior art audio power amplifiers are the amplifiers described in Hafler, 120 Watts of Hi-Fi Power, Radio & TV News, February 1959, pages 118-120, and Hafler, A 60-Watt "Ultra-Linear Amplifier", Radio & TV News, February 1955, page 45 et seq.
The problem with audio power amplifiers using the prior art circuit topology described in these references is that they possess certain performance deficiencies. Firstly, the direct-coupled voltage amplifier-phase inverter input circuit has poor current stability, so that variations in tube characteristics or tube supply voltages affect performance. Secondly, the phase inverter stage has limited drive capability to the power amplifier output stage, which results in higher distortion and less than optimum power output. The present invention overcomes these difficulties by utilizing a novel input circuit topology incorporating direct current feedback. Also included in the invention input circuit is a low impedance phase inverter having enhanced drive capability. The higher drive capability reduces distortion and also yields a higher power output by ensuring adequate drive power to the push-pull output stage.
A typical prior art input circuit partial schematic diagram is shown in FIG. 1. The voltage amplifier 11 is a triode, or pentode, vacuum tube operated as a high-gain voltage amplifier. The triode vacuum tube 13 is a well known split-load phase inverter, which has outputs of equal voltage but opposite phase at terminals 15 and 17.
FIG. 2 is a more detailed schematic diagram of a prior art input circuit. Resistor 21 is a grid leak resistor. The capacitor 23 is a bypass capacitor with reactance at least one order of magnitude lower than the resistance of screen-grid resistor 25 at the lowest audio frequency of 20 Hz. Resistors 25, 27, and 29 determine the direct current operating point for tubes 13 and 31. This operating point is generally set so as to cause the no-signal direct current voltage drops across tube 13 and both resistors 19a, 19b to be approximately equal. Stated another way, the voltage drop across each resistor 19a, 19b is equal to one-half the voltage drop across tube 13. As is well understood by those skilled in the art, this ensures maximum output voltage at the phase inverter output terminals 15 and 17.
Since no direct current feedback is employed from tube 13 to tube 31 in the circuit of FIG. 2, the direct current operating point of the circuit depends on the characteristics of pentode vacuum tube 31 and triode vacuum tube 13, as well as on the values of resistors 25, 27, and 29. The resistance values of resistors 25, 27 and 29 generally remain stable over time, but the characteristics of the tubes change with age. The aging causes a random drift of the direct current operating point of the circuit as the tubes age. The direct current operating point also drifts if filament or plate power supply voltages change, since a change in either aforementioned power supply voltage affects tube characteristics. Thus, the optimum operating point of the circuit would occur only by accident; usually an error exists which is sufficiently large to reduce circuit performance to less than optimum.
Another prior-art amplifier input circuit is shown in FIG. 3. This circuit differs from that shown in FIG. 2 because the circuit in FIG. 3 has the screen grid of pentode 31 connected through resistor 33 to the cathode of phase inverter tube 13 instead of to the B+ supply.
Resistor 21 is a grid leak resistor, capacitor 23 is a bypass capacitor with reactance at least one order of magnitude lower than the resistance of screen grid resistor 33 at the lowest audio frequency of 20 Hz. Resistors 27, 29 and 33 determine the direct current operating point for tubes 31 and 13. This operating point is generally set so as to cause the no signal direct current voltage across tube 13 and the sum of the direct current voltages across the two resistors 19a, 19b to be approximately equal
Direct current feedback from tube 13 to tube 31 is accomplished by connecting resistor 33 between the screen grid of 31 and the cathode of 13. Since this feedback connection unbalances the phase inverter outputs, a resistor 36 is added in series with the cathode load resistor 19b to compensate for the unbalance.
The mode of operation for the direct current feedback is explained hereinafter in the description of the preferred embodiments. Also explained is an improved means whereby the present invention overcomes the phase inverter unbalance problem of the prior art circuit of FIG. 3.